1. Field of the Invention
The present invention relates to pneumatic tires of radial construction provided with a carcass ply composed of steel cords and, more particularly, to an improved structure for reinforcing bead sections of large size radial tires for heavy vehicles, such as trucks.
2. Description of the Prior Art
Heretofore, in general, radial tires provided with a carcass ply composed of radially directed steel cords, and semi-radial tires wherein a carcass ply forms a small angle in the range of 15.degree.-30.degree. with respect to a radial direction of the tire, have fairly low rigidity in their bead sections when compared with that of bias ply tires, and this low rigidity in the bead section is largely to contribute to the improvement of riding comfort of radial tires. However, on the other hand, weak resistance to the lateral force against the tire, and hence a large deformation, is to invite a structural failure in the bead section within a short running period under severe service conditions.
In order to solve this technical problem which is peculiar to radial tires, various kinds of reinforcing structures of the bead section have previously been proposed.
For instance, the art which is disclosed in Japanese Patent No. 967,452 is well known to be one of outstanding reinforcing structure in the bead section. This patent, as shown in FIG. 2, is characterized in that the reinforcing layer 4, comprised of steel cords, is disposed at the outside of the turnup portion 2' of the carcass ply 2 and reinforcing layers 6, comprised of rubberized organic fiber cords wherein two or more layers are crossed with each other, are arranged at the outside of the aforesaid steel cord reinforcing layer 4 and extend radially upwardly from the bead base to the sidewall region. The rubber layers 11,12 are disposed between the carcass ply main portion 2 and the turnup portion 2' of the carcass ply 2 and extend outwardly radially from the bead core 3 and decrease in thickness towards the sidewall of the tire to form a substantially triangular section, which consists of a stiffener 11 of a hard rubber stock with a JIS (Japanese Industrial Standards) hardness of more than 80.degree., in a side of the carcass ply main portion and a buffer 12, composed of a relatively soft rubber stock with a JIS hardness of 50-60, in a side of the turnup portion 2' of a carcass ply 2 and extending along the organic fiber cord reinforcing layers 6 outwardly in a radial direction of the tire.
The tire constructed as above-described in FIG. 2 has been favorably accepted by drivers as the tire in which the bead durability is broadly improved, but recently some drivers have begun to use such tires under more even severe service condition, namely, such as running at high speed and a long period of running under heavy loads and high inflation pressure. Together with the increased use of radial tires under special heavy loads and high inflation pressure, has also come more frequent recapping in order to substantially reduce the cost of tires. It has become a tendency to recap a worn tire two or three times. Under such circumstances, the above-mentioned structure is insufficient for the requirement of durability of the bead portion, which is vital to preserve a worn tire for recapping. The above-described technical problem is one which the present invention seeks to remove.